This invention relates to an improved gas flow velocity and temperature sensor system and more particularly to a gas flow velocity and temperature sensing system utilizing a processor configured to use an empirically derived equation which more accurately calculates the gas flow velocity and temperature proximate a sensor.
Gas velocity and temperature sensors are used to monitor the gas (e.g., air) flow velocity and the temperature proximate sensitive electronic components, and also in refrigeration systems, gas conditioning systems, biocontainment systems, gas supply applications, industrial process control of gas mixing, weather applications and any application which requires monitoring of gas flow. For example, in electronic systems with heat generating components, failure to maintain sufficient gas flow within the system can result in damage to the sensitive electronics of the system. In biocontainment systems, failure to maintain the correct gas flow within the system can result in overheating or overcooling the biocontainment area killing the organisms within the system. In gas conditioning systems, gas supply applications, gas mixing, and weather applications, measuring the gas flow velocity and temperature within the system is key to operation of the system.
The inventors hereof have invented gas flow and temperature sensors/probes and circuits which facilitate easy access to even difficult locations by employing a small sensor comected over a long, flexible, small cross-sectional area of cable for providing a measurement of the gas flow velocity and temperature along with calibration data characterizing the response of the particular probe sensor and circuitry. See U.S. Pat. Nos. 5,929,333, 5,792,951, 5,511,415, and 4,733,541, incorporated herein in their entirety by these references.
These gas velocity sensors and probes, as well as other prior art gas velocity sensors/probes, typically employ two thermistors to calculate gas flow velocity and temperature. A thermistor is a thermally sensitive resistor which exhibits a change in electric resistance due to a change in temperature. One thermistor is typically maintained at the temperature of the gas flow being measured and a circuit connected to this thermistor is configured to output a temperature signal proportional to the gas temperature. The other thermistor is maintained at a chosen temperature which is significantly higher than the temperature of the gas being measured (e.g., a xe2x80x9chotxe2x80x9d thermistor 100xc2x0 C. above the temperature of the gas being measured). Because the resistance and temperature of a thermistor are related by a characteristic curve, a specific chosen temperature of the hot thermistor relates to specific resistance of the thermistor. A constant temperature servo connected to the hot thermistor maintains the hot thermistor at a constant resistance representative of the chosen temperature and outputs a measure of the power required to maintain the thermistor at the chosen resistance. When the hot thermistor is subjected to an increase or decrease in gas flow, it causes an increase or decrease in the power requirements of the constant temperature servo to maintain the hot thermistor at the constant resistance (representative of the chosen temperature). Typically, the constant temperature servo is configured to output a signal representative of the power dissipated as a function of gas velocity and the temperature proximate the thermistor.
Prior art gas flow velocity sensor systems may then employ a processor which receives the signal representative of the power dissipated as a function of gas velocity, the temperature signal representative of gas temperature proximate the hot thermistor, and the temperature signal representative of temperature of the gas being measured (often called the xe2x80x9cambient temperaturexe2x80x9d). The microprocessor of these prior art gas flow velocity sensors then calculates the gas flow velocity proximate the sensor using King""s law as shown below:                     P        =                                            E              v              2                        /                          R              v                                =                                    (                              Ak                +                                                                            Bk                      ⁡                                              (                                                                              μ                            ⁢                                                          xe2x80x83                                                        ⁢                                                          C                              P                                                                                k                                                )                                                              0.33                                    ⁢                                      Re                    n                                                              )                        ⁢                          (                                                T                  v                                -                                  T                  A                                            )                                                          (        1        )            
where P and Ev2/Rv is the power dissipated in a hot wire of infinite length, A, B, and n are constants derived via flow calibration, k is the fluid""s thermal conductivity, CP is heat capacity, Re is the Reynolds number, Tv is the temperature of the wire, and TA is the ambient temperature. The Reynolds number in expanded form is:       Re    =                  ρ        ⁢                  xe2x80x83                ⁢        vd            μ        ,
where p is the gas density, v is fluid (gas) velocity, d is the diameter of the wire, and xcexc is the gas viscosity. Equation (1) is solved for v to calculate the gas velocity as follows:                               gas          ⁢                      xe2x80x83                    ⁢          velocity                =                  v          =                                    K              2                        ·                                          [                                                      [                                                                  E                        v                        2                                            -                                                                        K                          0                                                ·                                                  (                                                      Tv                            -                            Ta                                                    )                                                                                      ]                                                                              K                      1                                        ·                                          (                                              Tv                        -                        Ta                                            )                                                                      ]                                            2                ⁢                                  xe2x80x83                                ⁢                37                                                                        (        2        )            
Based on the measured gas velocity, a feedback loop can be used to control gas velocity, and thus the temperature of, for example, an equipment cabinet or biocontainment system.
Prior art sensors and probes which rely on King""s law to calculate gas velocity, however, produce inaccurate readings because King""s law is based on the approximation that the hot thermistor is a hot filament of infinite length when in fact it is not.
It is therefore an object of this invention to provide an improved gas velocity and temperature sensor system.
It is a further object of this invention to provide such a sensor system which accurately measures the gas flow velocity and temperature proximate a sensor.
It is a further object of this invention to provide such a sensor system which refines the approximation used to calculate gas flow velocity.
It is a further object of this invention to provide such a sensor system which calculates gas flow velocity and temperature proximate a sensor without the errors associated with the approximation associated with King""s law.
The invention results from the realization that a truly effective gas flow and temperature sensor can be effected by providing a first thermistor driven at a constant temperature higher than the gas temperature being measured and which outputs a signal representative of the power dissipated as a function of gas velocity, a second thermistor which measures the gas temperature and which outputs a signal representative of the gas temperature, and a microprocessor configured to calculate a more accurate representation of the gas flow velocity and temperature, not by using King""s law which relies on the erroneous approximation that the thermistor is a hot wire of infinite length, but, instead, by utilizing a innovative and significantly more accurate empirically derived equation which reduces the error of approximation associated with King""s law to yield a significantly more accurate measurement of gas flow velocity and temperature proximate the sensor.
This invention features a gas velocity and temperature sensor system comprising a first thermistor driven at a constant temperature and configured to output a flow signal representative of the power dissipated as a function of the gas velocity and a temperature signal representative of the temperature of the first thermistor, a second thermistor configured to output a gas temperature signal representative of the gas temperature proximate the second thermistor, and a processor responsive to the flow signal and the temperature signals. The processor is configured to calculate gas velocity using an empirically derived equation in which gas flow velocity is a function of a constant and the ratio of the power dissipated to the temperature difference between the temperature of the first thermistor and the gas temperature proximate the second thermistor, the processor deriving a signal representing the gas velocity. Ideally, the processor derives a signal representing the temperature of the gas proximate the second thermistor.
In one preferred embodiment the empirically derived equation is   v  ≅            [              kP                  Δ          ⁢                      xe2x80x83                    ⁢          T                    ]              5      /      2      
where k is the constant representing calibration constants of the first and second thermistors, P is the power dissipated, and xcex94T is the difference between the temperature of the first thermistor and the gas temperature proximate the second thermistor.
In one example of this invention, a non-volatile memory is configured to store the constant k accessible and readable by the processor to calculate the gas velocity from the empirically derived equation. Typically, the processor stores the constant k accessible to calculate the gas velocity from the empirically derived equation. Preferably the flow signal and the temperature signals are voltages, but alternatively the flow signal and the temperature signal may be currents.
In one design of this invention, a constant temperature servo may be connected between the first thermistor and the processor to drive the first thermistor at a constant resistance equal to a predetermined constant temperature. Typically, an amplifier circuit may be connected between the second thermistor and the processor to amplify the gas temperature signal output by the second thermistor. Ideally, an analog-to-digital converter is connected between the constant temperature servo and the processor configured to convert the flow signal and the temperature signal of the first thermistor to a digital flow signal and a first digital temperature signal. The gas velocity sensor system of this invention may also include an analog-to-digital converter connected between the amplifier circuit and the processor configured to convert the gas temperature signal to a second digital temperature signal. In one embodiment, the gas velocity sensor system of this invention includes a digital-to-analog converter connected between the processor and an output drive circuit configured to convert the signal representing the gas flow velocity and the signal representing the temperature of the gas derived by the processor to an analog flow signal and an analog temperature signal. In a preferred example, the drive circuit is configured to condition the analog flow signal and analog temperature signal to be output in the range of 0-5 volts, or alternatively in the range of 0-10 volts. In another example, the drive circuit may be configured to condition the analog flow signal and analog temperature signal to be output in the range of 0-20 milliamperes, or 4-20 milliamperes.
This invention also features a gas velocity and temperature sensor system comprising a first thermistor driven at a constant temperature and configured to output a flow signal representative of the power dissipated as a function of the gas velocity and a temperature signal representative of the temperature of the first thermistor, a second thermistor configured to output a gas temperature signal representative of the gas temperature proximate the second thermistor, and a processor responsive to the flow signal and the temperature signals. The processor is configured to calculate gas velocity using an empirically derived equation in which gas flow velocity is the function of a constant and the ratio of the power dissipated to the temperature difference between the temperature of the first thermistor and the gas temperature proximate the second thermistor. The processor derives a signal representing the gas flow velocity and a signal representing the temperature of the gas.
This invention also features a gas velocity and temperature sensor system comprising a first thermistor driven at a constant temperature and configured to output a flow signal proportional to the power dissipated as a function of the gas velocity and a temperature signal representative of the temperature of the first thermistor, a second thermistor configured to output a gas temperature signal proportional to the gas temperature proximate to the second thermistor, and a processor responsive to the flow signal and the temperature signals configured to calculate gas velocity using the empirically derived equation:       v    ≅                  [                  kP                      Δ            ⁢                          xe2x80x83                        ⁢            T                          ]                    5        /        2              ,
where k is a constant, P is the power dissipated as a function of the gas velocity, and xcex94T is the difference between the temperature of the first thermistor and gas temperature proximate to the second thermistor, the processor deriving a signal representing the gas velocity.
Ideally, the processor derives a signal representing the temperature of the gas. Typically, a non-volatile memory configured to store the constant k which represents the calibration coefficients of the first and second thermistors, the memory readable by the processor to calculate the gas velocity from the empirically derived equation.
In one design, the gas velocity and temperature sensor system of this invention includes a first thermistor driven at a constant temperature and configured to output a flow signal representative of the power dissipated as a function of the gas velocity and a temperature signal representative of the temperature of the first thermistor, a second thermistor configured to output a gas temperature signal representative of the gas temperature proximate to the second thermistor, a processor responsive to the flow signal and the temperature signals configured to calculate the gas velocity proximate the sensor using the empirically derived equations:                     x        =                                            k              0                        ⁢                                                            (                                      F                    -                                          F                      0                                                        )                                2                                            Δ                ⁢                                  xe2x80x83                                ⁢                T                                              -                                    k              1                        ⁢            Δ            ⁢                          xe2x80x83                        ⁢            T                    -                                    k              2                        ⁢                          T              A                                +                                    k              3                        ⁢                          xe2x80x83                        ⁢            and                                                                        gas            ⁢                          xe2x80x83                        ⁢            velocity                    =                                                    k                4                            ⁢                              x                                  k                  7                                                      +                                          k                5                            ⁢                              1                                  x                  2                                                      +                          k              6                                      ,            
where x is the compensated power, F is the power dissipated in the first thermistor as a function of the gas velocity, F0 is the quiescent power of the first thermistor, xcex94T is the difference between temperature of the first thermistor and the gas temperature proximate the second thermistor, k0, k1, k2, k3, k4, k5, and k6 are calibration constants of the first and second thermistors, and k7 is an empirically derived constant, and the processor deriving a signal representing the gas velocity and a signal representing the temperature of the gas.
In the preferred embodiment, k7 is 2.33. Typically, a non-volatile memory is configured to store k0, k1, k2, k3, k4, k5, k6 and k7 calibration coefficients readable by the processor and used to calculate the gas velocity from the empirically derived equation.
This invention further features a gas velocity and temperature sensor system comprising a processor responsive to a flow signal from a first thermistor representative of the power dissipated as a function of the gas velocity and temperature of the first thermistor and a gas temperature signal from a second thermistor representative of the gas temperature proximate to the second thermistor, the processor configured to calculate the gas velocity using an empirically derived equation which is a function of a constant and the ratio of the power dissipated to the temperature difference between the temperature of the first thermistor and the gas temperature proximate the second thermistor. The processor derives a signal representing the gas flow velocity. Ideally, the processor derives a signal representing the temperature of the gas proximate the second thermistor.
In another design, the gas velocity and temperature sensor system of this invention comprises a processor responsive to a flow signal from a first thermistor representative of the power dissipated as a function of the gas velocity and temperature of the first thermistor and a temperature signal from a second thermistor representative of the gas temperature proximate to the second thermistor. The processor is ideally configured to calculate the gas velocity using an empirically derived equation which is function of a constant and the ratio of the power dissipated to the temperature difference between the temperature of the first thermistor and the gas temperature proximate the second thermistor, the microprocessor deriving a signal representing the gas velocity and a signal representing the temperature of the gas.
This invention also features a method for measuring the gas velocity and temperature, the method includes driving a first thermistor at a predetermined constant temperature, detecting a signal representative of the power dissipated as a function of the gas velocity of the first thermistor and a temperature signal representative of the temperature of the first thermistor, detecting a signal representative of the gas temperature proximate a second thermistor and calculating the gas velocity using an empirically derived equation in which the gas flow velocity is a function of a constant and the ratio of the power dissipated to the temperature difference between the temperature of the first thermistor and the temperature proximate the second thermistor.
In one embodiment, a constant temperature servo drives the first thermistor at a predetermined constant temperature. Ideally, the constant temperature servo provides the signal representative of the power dissipated as a function of gas velocity and the temperature of the first thermistor. Preferably, an amplifier circuit detects the signal representative of the gas temperature. Typically, a processor calculates the gas velocity using the empirically derived equation. In one embodiment, the method for measuring gas velocity and temperature may include the step of storing the constant in a memory accessible and readable by the processor, and the step of converting the signal representative of the power dissipated as a function of gas velocity output by the constant temperature servo to a digital flow signal and converting the temperature signal representative of the temperature of the first thermistor to a first digital temperature signal. Ideally, an analog-to-digital converter converts the signal representative of the power and temperature of first thermistor to a digital flow signal and a first digital temperature.
In one example, the method further includes the step of converting the signal representative of the gas temperature proximate the second thermistor to a second digital temperature signal. Typically, an analog-to-digital converter converts the gas temperature proximate the second thermistor to a second digital temperature signal.
The method of measuring gas velocity and temperature of this invention may further include the step of converting the signal representative gas velocity calculated by the processor to an analog flow signal and converting the temperature signal representative of the temperature of the first thermistor to an analog temperature signal and also further include the step of converting the signal representative of the gas temperature proximate the second thermistor output by the processor to an analog temperature signal. In one embodiment, a digital-to-analog converter converts the digital signal representative of the gas velocity and digital signal representative of the gas temperature to an analog flow signal and an analog temperature signal.
This invention further features a method for measuring the gas velocity and temperature, including the steps of driving a first thermistor at a predetermined constant temperature, detecting a flow signal representative of the power dissipated as a function of the gas velocity of the first thermistor and a temperature signal representative of the temperature of the first thermistor, converting the flow signal and temperature signal to a digital flow signal and a first digital temperature signal, subtracting the quiescent power from the power dissipated in the first thermistor, measuring the gas temperature with a second thermistor configured to output a signal representing the gas temperature proximate the second thermistor, converting the signal representing the gas temperature to a second digital signal, subtracting the second digital temperature signal from the first digital temperature signal, deriving the digital gas temperature signal, calculating the compensated power x, using the equation   x  =                    k        0            ⁢                                    (                          F              -                              F                0                                      )                    2                          Δ          ⁢                      xe2x80x83                    ⁢          T                      -                  k        1            ⁢      Δ      ⁢              xe2x80x83            ⁢      T        -                  k        2            ⁢              T        A              +          k      3      
calculating the gas flow velocity using the equation gas velocity             gas      ⁢              xe2x80x83            ⁢      velocity        =                            k          4                ⁢                  x                      k            7                              +                        k          5                ⁢                  1                      x            2                              +              k        6              ,
where x is the compensated power, F is the power dissipated in the first thermistor as a function of the gas velocity, F0 is the quiescent power of the first thermistor, xcex94T is the difference between temperature of the first thermistor and the gas temperature proximate the second thermistor, k0, k1, k2, k3, k4, k5, and k6 are calibration constants of the first and second thermistors, and k7 is an empirically derived constant; and deriving a signal representing the gas flow velocity and a signal representing the gas temperature.
This invention also features a gas velocity and temperature sensor system comprising a power dissipated and temperature sensing means driven at a constant temperature for outputting a flow signal representative of the power dissipated as a function of the gas velocity and a temperature signal representative of the temperature of the first power dissipated and temperature sensing means, a temperature sensing means for outputting a gas temperature signal representative of the gas temperature proximate the temperature sensing means, and means responsive to the flow signal and the temperature signals, for calculating gas velocity using an empirically derived equation in which gas flow velocity is function of a constant and the ratio of the power dissipated to the temperature difference between the temperature of the first thermistor and the gas temperature proximate the second thermistor. The means responsive to the flow signal and temperature signal derives a signal representing the gas velocity. Ideally, the power dissipated and temperature sensing means is a first thermistor, the temperature sensing means is a second thermistor, and the means responsive to the flow signal and temperature signal is a processor.